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Week 4 – Drops on a Penny
Today’s Topic: Learning how a scientist does their job—
specifically counting accurately
Note: We recommend this experiment be performed by individual
“Scientists-in-Training.”
Teacher Summary:
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Students should be exposed to the different kinds of scientists in
our world.
Students should understand that scientists must count accurately
as one part of their job.
Student Objectives:
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Students should be able to use a pipette.
Students should make a hypothesis about how many drops of
water they can fit on a penny.
• Students will count the number of drops accurately.
• Have fun!
Materials for 20 students:
• 20 plastic pipettes
• 20 pennies
• 10 cups of water
• 20 paper towels
Materials per student:
• 1 plastic pipette
• 1 penny
• Cup of water shared with 1 other student
• 1 paper towel
Room Setup
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•
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Place one paper towel at each student’s seat.
Fill 10 cups ¼ full of water and set aside as they will be handed out later.
Lay out 20 pennies and 20 pipettes to be handed out later.
Lay out student lab coats, hardhat and goggles.
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Opening
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Select your Scientist-in-Training and Engineer-in-Training helpers. Have them come up and put
on their lab coat, hardhat and goggles.
Hand out Student Binders.
Lead or choose a student to lead the JESS Promise.
Introduce the Experiment
Terms to Tackle
Scientific Method—a method of research used by scientists to come to a definitive conclusion
Hypothesis—prediction or guess concerning the outcome of a scientific experiment
Pipette—a slender tube used in a laboratory to measure and transfer fluids from one container to
another
Teacher Introduction:
1. Tell the students that today they are going to become scientists just like they read about in the
book, What is a Scientist?
2. Explain that they are going to count drops of water on a penny because it is very important that
scientists count accurately when they are performing an experiment.
3. Hold up a pipette and talk about what it is and how it works. Squeeze the bulb, place the bulb in
the water, and release the bulb so water is sucked up into the pipette.
4. Introduce the word “hypothesis” and ask if anyone know what it means. Explain that a hypothesis
is a prediction or guess about what will happen when they perform their experiment.
5. Make a table on the board. It should have 3 columns labeled:
Name
Hypothesis
Results
6. Select 7-8 students to give you their hypotheses about the maximum number of drops of water
they think they will be able to drop on their penny before the surface tension breaks and they
have to start over. For now, just fill in the columns labeled Name and Hypothesis.
Perform the Experiment
1. Have your helpers’ hand out the paper towels, pipettes and pennies to each student. See photo
on page 38
2. Tell students to place their pennies on the paper towel.
3. Show students how to very gently squeeze the pipette bulb over the penny to lightly drop one
drop of water at a time onto the penny. Let them practice on their own pennies. See photo on
page 39 for reference.
4. Students should count the number of drops they can put onto the penny until the surface tension
breaks (the bubble of water breaks) and then they should move the penny to a dry spot on the
paper towel and begin again. See photo on page 39.
5. If time allows, have them try different variables like:
• Heads vs. tails
• Clean penny vs. “dirty”
• Dime
• Nickel
• Quarter
6. Remember to fill in the table on the board with the results.
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What Just Happened?
When you place water (H20) drops on a penny, the drops pile up into a small dome
because water molecules are attracted to each other in all directions, making them
“stick” together (see photo on page 39). However, the molecules at the surface
“stick” only to the molecules next to and below them. That’s because there are
none above them. This makes the surface act as if it had a thin “skin.” This is
called surface tension. As you add more drops, the force of gravity becomes
stronger than the force of attraction among the water molecules at the surface.
This causes the water to spill over the edge of the coin.
Wrap it Up!
Query the students as to what they learned:
1.
2.
3.
4.
What was your hypothesis about how many drops of water you would fit on your penny?
What was the actual number you were able to fit?
Do you think you could fit more or less drops of water on a dime? A nickel? A quarter?
What could have caused one person to fit more drops of water of their penny than yours?
(potential answer: smaller opening at the tip of their pipette, they dropped smaller drops, their
penny was cleaner, etc.)
Bibliography:
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Lehn, Barbara, What is a Scientist?, Minneapolis, Millbrook Press, 1998.
http://pbskids.org/zoom/activities/sci/dropsonpennies.html
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Photos and Illustrations
Counting Drops on a Penny
Simple setup.
Carefully placing one drop at a time on the penny.
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Water sticks to the penny.
If you are careful you can get a lot of water on the penny.